The invention takes a hydraulic steering system as its starting-point, which is used, in particular, for a mobile working machine.
DE 43 14 616 A1 has disclosed a hydraulic steering system that has a first steering member, which is constructed in the manner of a steering wheel and by means of which a steering unit with a first fluid connection, which is connected to a pressure chamber of a steering motor, said pressure chamber being subjected to pressure for a steering movement to the left, and with a second fluid connection, which is connected to a pressure chamber of the steering motor, said pressure chamber being subjected to pressure for a steering movement to the right, can be actuated. The steering unit comprises, in a known manner, a metering pump and a valve and returns to a neutral position once the rotation of the steering member is finished. The steering unit furthermore allows hydraulic actuation of a control valve, the control chambers of which, which can be subjected to pressure for actuation, can be connected by respective shuttle valves to the working lines leading from the steering unit to the steering motor. In the steering system disclosed in DE 43 14 616 A1, the steering motor furthermore comprises two hydraulic cylinders.
In the steering system disclosed in DE 43 14 616 A1, the control valve can also be actuated by means of a second steering member, which is constructed in the manner of a joystick. This contains two pressure-reducing valves, it being possible for the outlet of one pressure-reducing valve to be connected to one control chamber via one shuttle valve and the outlet of the other pressure-reducing valve to be connected to the other control chamber of the control valve via the other shuttle valve. A connection is in each case established when the control pressure at the outlet of a pressure-reducing valve is higher than the pressure in the working line extending between the steering unit and the steering motor and connected to the same shuttle valve. The two working connections of the control valve are connected to two other pressure chambers of the steering motor.
The two pressure-reducing valves of the second steering member are supplied with control oil from a feed line, in which a substantially constant supply pressure prevails. The feed line leads via a 2/2-way selector valve, which is controlled by the pressures at the two fluid connections of the steering unit and interrupts the feed line when there is a pressure in one of the two fluid connections. This is intended to ensure that steering with the first steering member has priority over steering with the second steering member.
A hydraulic steering system is known from DE 40 31 951 A1. In this hydraulic steering system, the control valve is associated only with the second steering member and cannot be actuated by means of the first steering member. The steering motor is a synchronous-motion cylinder with two pressure chambers, each of which is connected either to a working connection of the steering unit or to a working connection of the control valve. To ensure priority for a steering actuation by means of the first steering member over a steering actuation by means of the second steering member, by means of which the control valve is activated, the two pressure chambers of the steering motor have arranged upstream of them valves that can be moved by the pressure at a working connection of the control valve into a position such that the working connection is connected to the corresponding pressure chamber of the steering motor. A control line, for which the control oil is taken from the respectively pressurized working connection of the control valve, can be relieved to a tank by means of a further valve, which is operated by the pressure at a load-signaling connection of the steering unit. Thus, as soon as a pressure is applied to the load-signaling connection of the steering unit, the control line is relieved and the selector valves at the pressure chambers of the steering motor move into a position in which the connection between the working connections of the control valve and the pressure chambers is shut off and a connection is established between the working connections of the steering unit and the pressure chambers.
It is an object of the invention to ensure priority of a steering actuation by means of the first steering member over a steering actuation by means of the second steering member in a simple manner in a hydraulic steering system.
According to the invention, this object is achieved in that the control valve can only be activated when the steering unit is in the neutral position. Thus, the control valve cannot then be moved out of a neutral central position, making it unnecessary also to have an expensive means of shutting off its working connections from the steering motor. There is an intervention only in the pilot-control circuit of the control valve, in which the pressures are low and the quantities of oil to be controlled are small.
Advantageous refinements of a hydraulic steering system according to the invention are also provided. The pilot-control signal can usually be generated from a supply signal by pilot-control means adjustable by means of the second steering member. In principle, it is conceivable to intervene in the control lines downstream of the pilot-control means in order to prevent actuation of the control valve. Since there are generally two such control lines, it would be necessary to intervene in both. It seems simpler to switch off the supply signal fed to the pilot-control means.
Nowadays, pilot-controlled directional control valves are predominantly actuated hydraulically or electro-hydraulically. In both cases, the control valve has two control chambers, which can be subjected to control pressure, the directional-control spool of the control valve then being displaced out of a central position against a centering-spring arrangement. If the control valve is adjustable in a proportional manner, the control chambers can be subjected to different control pressures. According to a refinement of the invention there is a cut-off valve, which, in a position that it adopts in the case of priority, interrupts a connection of the pilot-control valves to a control-pressure supply line. In particular, according to another refinement, the cut-off valve relieves its outlet connected to the pilot-control valves to a tank in one position, with the result that the control chambers in the control valve are also relieved of pressure and the directional-control spool of the control valve returns to its central position. All further steering movement due to supply of fluid to the steering motor via the control valve ceases.
So-called load-sensing steering units have a pressure-signaling connection, at which a pressure signal, in particular the load pressure applied to the first or second fluid connection, can be picked off when the steering unit has been adjusted out of the neutral position. According to a feature of the invention, it is advantageously possible for a pilot-control signal to be generated or not by means of the second steering member as a function of this pressure signal. In particular, according to another feature of the invention the selector valve, which, in one position, interrupts a connection of the pilot-control valves to the control-pressure supply line, can be switched over hydraulically directly by the pressure applied to the pressure-signaling connection of the steering unit.
The pilot-control valves can be manually actuable or, alternatively, actuable electrically, in particular by an electromagnet. In the case of electric actuation, the pilot-control means are, according to still another refinement of the invention, advantageously switched off electrically. That is to say, the electric actuator of a pilot-control valve does not receive an electrical control signal if the steering unit is not in its neutral position, despite the fact that the second steering member is actuated. This appears to be a simple solution, especially when the hydraulic system of a mobile working machine in any case already contains a large number of electronic components that also intervene in the steering system.
In such a case, it is superfluous to divert away the supply pressure for the pilot-control valves. However, safety reasons may argue for additionally diverting away the supply pressure as well. The means used for switching off electrically is advantageously a pressure switch, which can be actuated by the pressure applied to the pressure-signaling connection of the steering unit.
According to still another refinement of the invention, provision is made for a movement of the first steering member to be detected by an electric signal transmitter and for the pilot-control means to be capable of being switched off by means. of the electric signal transmitter. A construction of this kind is of universal application, even in those steering units which do not have a pressure-signaling connection. Moreover, this solution is, in turn, particularly simple in systems in which a lot of electronics are used.
Particularly for mobile working machines, the hydraulic system of which also comprises electronic controls, e.g. in the form of a microcomputer or microcontroller, it is easy to make the steering rate, which is dependent on the actuating travel of the second steering member in the case of proportional activation of the control valve, dependent on the speed of the vehicle too. For example, the adjustment travel of the control valve can be made progressively smaller as the speed increases by intervention of the microcontroller at a given position of the second steering member. Another and more simple possibility of allowing for the driving speed in the steering rate is, according to a feature of the invention, for only the maximum possible magnitude of the pilot-control signal to be variable as a function of the vehicle speed. This means that, when the second steering member is actuated out of its neutral position, the vehicle speed initially has no effect on the steering rate. After a certain actuating travel, however, the pilot-control signal for the control valve can no longer be increased, even if the second-steering member is moved further. The point at which the control valve can no longer be adjusted further now depends on the vehicle speed. At a high vehicle speed, it is closer to the neutral position of the second steering member than at low vehicle speed. Sharp steering corrections with the second steering member are thus even less possible, the higher the vehicle speed. At the same time, the control can be configured in such a way that the reduction in the maximum steering rate occurs only above a certain minimum speed. The vehicle speed can be detected by means of a speedometer. However, mobile working machines are generally also fitted with driving hydraulics, and a control pressure for the driving hydraulics is a measure of the vehicle speed. This control pressure can now be used to activate a valve by means of which a maximum possible pilot-control pressure can be varied.
A joystick steering facility can also be used on so-called excavator loaders. Nowadays, the pilot-control devices are generally integrated into the rests of the vehicle seat. According to still another feature, the adjustment of the control valve as a function of the actuating direction of the second steering member is reversed by rotation of the driver""s seat through 180 degrees, with the result that steering takes place in the appropriate manner in both directions of travel of the excavator loader.